Provider is being configured on class level only

[Nazar Matus]

The issue is the same as this one on another gem: elastic/elasticsearch-rails#811

This means that if I need some dynamic providers - I have to define an extra class that is going to build a new provider class every time. That's a current workaround.

But if you want to improve this gem - you might want to allow provider configuration on instance level as well, for v2 for example :)